The present invention relates to a material and method for filling and otherwise restoring human teeth.
Historically, dentists have utilized various metals and metal amalgamations to fill cavities and otherwise replace missing or removed tooth structure. However, tooth restoration with such metals does have certain deficiencies, one of the principal deficiencies being the aesthetic appearance of the metal.
Numerous organic compounds have also been used in a variety of mixtures and proportions in an attempt to discover materials for dental composites and restoratives that would have improved characteristics as compared to the metals. Dental composite and restorative materials must not only have good forming characteristics in order to be shaped to fit a cavity or be molded in place to repair chipped or damaged teeth but must also have physical and chemical properties which are compatible with a dental, physiological environment, such as thermal coefficient of expansion, non-toxic, insoluble, non-corrosive, etc.
Further, such restorative compositions must exhibit satisfactory hardness and durability characteristics in order to meet the requirements of their intended purpose. In attempts to satisfy the hardness and durability characteristics, researchers have utilized various compounds often including some type of resin integrated with various amounts of particulate material, such as organic polymers, various kinds of metals, ceramics, and the like. Such compounds usually also include other materials, such as pigments, catalysts, handling agents and opacifiers, and the like, for aesthetic and other purposes.
When such restorative materials are placed in the mouth, the materials must securely bond both to itself integrally and to a tooth being restored. Since the final bonding occurs during installation in the mouth, it is particularly essential that satisfactory adhesion be obtainable in the environment of the mouth.
One method which has been utilized to enhance securement of composites and adhesives to tooth enamel is to severely etch the enamel with a highly acidic solution. This procedure intentionally demineralizes the enamel and creates deep pits and irregularities in the surface of the enamel to provide mechanical interlocking with the underlying tooth structure as the primary means of retention of the restorative. Although such etching of the underlying tooth structure for bonding purposes usually does provide adequate adhesion, it also causes deteriorative effects on the underlying tooth structure.
Another method which has helped alleviate such destructive treatment of the underlying tooth structure is the utilization of an adhesive, such as a glass ionomer cement which bonds strongly and directly to the tooth structure. Glass ionomer cements consist of a particulate glass powder and a mixing fluid which may generally be described as an aqueous solution of a polycarboxylic acid. The diameter range of particles in the glass powder must be suitable for dental applications. The particle size and size distribution of the glass particles can be adjusted using conventional techniques, such as by grinding, screening, sedimentation or other particle classification methods. Control of the range and distribution of particle size is an important characteristic for influencing the strength, work time and set time of the cement.
Work time and set time can also be adjusted by affecting the surface area of the glass particles, such as by etching with an acid and thoroughly washing the treated glass to leave substantially no soluble calcium salts on the surface of the glass particles.
One process for making glass ionomer cement powder involves comminuting carboxylic acid with a chemically active glass, such as an aluminosilicate glass which has been prepared with a fluoride flux. The work time and set time of the cement may be influenced by the molecular weight and carboxyl equivalent weight of a particular polyacid or by the relative quantity of carboxylic acid added during the comminution step. For example, a low relative quantity of carboxylic acid, such as approximately 3% by weight or less, will extend work time without substantially affecting set time. Such characteristics are generally desired for luting cements, veneer cements or orthodontic bracket adhesives. Similarly, a larger relative quantity of carboxylic acid, such as approximately 5% by weight or more, will extend both work time and set time. Such characteristics are generally desired for endodontic sealants or bone cements and for applications where high glass loading levels are desired, such as for basing cements, crown build-up cements or posterior liners.
In addition to controlling particle size distribution and area of the glass, selection of a polyacid, and adjusting the ratio of glass to polyacid, work time and set time of the cement may be further adjusted by the addition of a chelating agent, such as tartaric acid or the like.
To form the glass ionomer cement, the glass particles and acid are comminuted under sufficiently vigorous, substantially anhydrous pulverization conditions, such as by ball milling, to cause reaction between the glass and carboxylic acid such that carboxylate salt is formed in the glass powder. The comminution must be conducted under substantially anhydrous conditions as the presence of moisture can result in the formation of carboxylate salt having entrained water, which results in poor mix properties and shortened work times. The comminuted glass and carboxylic acid forms a water-hardenable cement.
When actually using the glass ionomer cements to restore a tooth, a solvent, such as water, is added to the cement, whereupon multivalent ions, such as calcium ions, leach from the glass and cross-link the carboxylic acid chains during curing which results in formation of the restorative composition. The mixture undergoes a brief working period, during which the reactants are converted from a creamy paste to a relatively firm, carvable solid. The working period is followed by a brief setting period, during which the carvable solid becomes sufficiently strong to function as a dental cement.
Glass ionomer cements have generally enjoyed widespread application since they generally exhibit excellent adhesion characteristics to calcified tooth structure, including both enamel and dentin tooth substances. Besides superior adhesion characteristics, glass ionomer cements also excel in marginal sealing and durability in the mouth over a long period of time. In addition, glass ionomer cements generally exhibit little or no irritant action, detrimental corrosion, or other harmful pathological action upon the dental pulp. Further, glass ionomer cements maintain excellent resistance to the mouth tissues or fluids over extended periods of time.
Unfortunately, however, glass ionomers have not been particularly useful for certain applications. For example, glass ionomers by themselves are visually inferior to composite resins. As a restorative, glass ionomers are extremely sensitive to technique and usually are not polishable. Glass ionomer cements are usually overly brittle which limits their use in the molar region and at corners and edges of a tooth.
Further, a glass ionomer generally has a relatively weak cohesive strength. As a result, the glass ionomer bonds more strongly to the underlying tooth structure than it bonds to itself. This is sometimes observed where a filling comprising a glass ionomer fails; the failure occurs within the bulk of the glass ionomer while the bond between the tooth and the glass ionomer remains intact.
To compensate for some of the deficiencies of glass ionomer cements, the cement powder can contain or be combined with appropriate quantities of viscosity modifiers such as microfine silica, wetting agents, milling agents, extending fillers, radiopacifiers, metal powders such as silver or silver alloys, medicants, and the like. In addition, such a composition may include materials having beneficial aesthetic properties such as adjuvants including pigments for matching those of natural healthy human teeth, plaque repellency, polishability and opacity.
Another approach used to prepare a composition for restoring teeth involves imbedding glass particles in a binder, such as methyacrylate which usually achieves good bonding characteristics with the underlying tooth structure. Unfortunately, the methyacrylate does not simultaneously achieve acceptable bonding with the glass particles imbedded therein unless they have been subjected to acid etching.
A suitable, self-adhering dental restorative should preferably provide certain beneficial attributes at the juncture between the restorative and the abutting tooth structure or at the exposed surfaces thereof. Such attributes include availability of leachable calcium, availability of leachable fluoride to minimize the formation of secondary caries, sealing characteristics to minimize microleakage by providing a substantially impervious protective barrier, hydrophilic characteristics sufficient to adequately wet dentine in vivo, an ability to bond to both dentin and enamel, natural appearance, optimal placement consistency, substantially pH-neutrality for maximum healing potential, extremely low solubility and disintegrability, and non-bioresorbability. One of the principal benefits of glass ionomer cement or methacrylate lies in the fact that such materials for tooth restoratives can be placed directly on or into a human tooth without any underfilling or other similar measures and obtain a result which is physiologically satisfactory cosmetically and mechanically and substantially meet the above noted criteria.
The major drawback in using glass ionomer cement or methacrylate in tooth restoration is that such compositions tend to wear relatively quickly when used in locations where teeth engage or where wear otherwise frequently occurs.
There is a definite need for a dental filling and sealing composition which requires minimal removal of healthy tooth structure while providing a strong, permanent, long-wearing restoration having a pleasing, natural appearance. Such a composition should possess good bonding internally as well as with the underlying tooth structure and should possess structural properties which closely match those of natural healthy teeth, such as cohesive strength, wearability, coefficient of thermal expansion and durability.